Glyphosate-resistant ryegrass, one hard-to-kill weed

Glyphosate-resistant biotypes of Italian ryegrass have been documented in Mississippi, Arkansas and North Carolina.
The best timing for control of glyphosate-resistant Italian ryegrass is before it ever comes up.
There are very few postemergence options for control of glyphosate-resistant Italian ryegrass, and once it's in crop, there's nothing producers can do.

Glyphosate-resistant Italian ryegrass may not be as prolific a seed producer as glyphosate-resistant pigweed. Nor does it spread as ferociously. But in many fields, it can be just as hard to kill.

Weed scientists from three states discussed the weed in March, at the Glyphosate-Resistant Ryegrass Field Day at the Delta Research and Extension Center.

Twelve counties in Mississippi are currently the epicenter of glyphosate-resistant Italian ryegrass, but resistant biotypes have been discovered also in Arkansas and North Carolina.

DREC weed scientist Jason Bond says glyphosate-resistant Italian ryegrass shares another control characteristic with glyphosate-resistant pigweed. The best time to control it is before it ever comes up. But Bond believes that Mid-South farmers can bring glyphosate-resistant Italian ryegrass under control “if we really hammer it for a couple of years in a row.”

At the field day, which was moved indoors due to muddy conditions, Bond and fellow DREC weed scientist Tom Eubank described their research and recommendations for control of glyphosate-resistant Italian ryegrass.

Fall recommendations: For corn, Dual Magnum at 1.33 pints per acre; cotton, Dual Magnum at 1.33 pints per acre, or Treflan incorporated at 3 pints per acre, or double disking; soybeans, Dual Magnum at 1.33 pints per acre, or Treflan incorporated at 3 pints per acre, or double disking; and rice, Command at 2 pints per acre.

“The best timing for a fall application is from mid-October to mid-November,” Bond said. “None of these products are going to have post emergence activity on ryegrass. So if there is any ryegrass up, you have to kill it before you put a pre-emergence down or you’re going to be extremely disappointed in the spring.”

Winter application: Herbicide applications should be based on careful scouting for emerged glyphosate-resistant Italian ryegrass. Bond and Eubank suggest Select Max at 12 ounces to 16 ounces, or an equivalent rate of 2 pounds of clethodim for fields going to corn, soybean, cotton or rice. The best timing for winter applications are from mid-January to mid-February, when Italian ryegrass is less than 6 inches tall. Preplant applications of Select Max should be made at least 30 days prior to planting corn or rice. The higher rate of Select Max should be used if no residual herbicide was applied in the fall or if used in combination with 2,4-D or dicamba.

“Select does look good,” said Eubank. “But it is under the ACCase umbrella and we do have resistance to this family of chemistry. So we don’t need to depend on it too much. If we continue to put pressure on this herbicide, we will see resistance.”

Spring applications: Eubank said paraquat (Gramoxone Inteon) “is the best product we have right now for emerged Italian ryegrass, but control may be poor on larger plants. Spring applications of Gramoxone Inteon at 3 pints to 4 pints or two applications of Gramoxone Inteon spaced 10 to 14 days apart are recommended for cotton, corn, soybean and rice. A spring application should be made from March 1 to March 20 based on careful scouting for emerged glyphosate-resistant Italian ryegrass.

Research indicates that the addition of atrazine in corn at 1 quart, Sencor in soybean at 4 ounces, or Direx in cotton at 1.5 pints will increase the efficacy of Gramoxone Inteon against emerged glyphosate-resistant Italian ryegrass.

Spray coverage

Spray coverage is also critical, weed scientists say. Be sure to use spray nozzles such as flat fan or twin jet to ensure thorough coverage of the weed. Avoid the use of air induction nozzles with contact herbicides.

Eubank says a 6-inch tall plant with 6 tillers “is the breaking point for control. You’re not going to control it very effectively after this point. It is probably going to require a two-pass program to effectively control Italian ryegrass — either a fall residual followed by a winter application of clethodim or a winter application of clethodim followed by a spring application of paraquat.”

Eubank noted that there are very few postemergence options for control of glyphosate-resistant Italian ryegrass. “Once you’re in crop, there is nothing you can do. So it’s imperative that glyphosate-resistant Italian ryegrass is controlled prior to planting.”

Arkansas weed scientist Bob Scott said herbicide-resistant Italian ryegrass “is the No. 1, 2 and 3 problems in the state’s wheat crop.” Scott says most ryegrass populations are resistant to Hoelon and some are resistant to ALS herbicides, while glyphosate-resistant Italian ryegrass “is becoming a bigger and bigger problem.”

Alan York, a weed scientist at North Carolina State University, says glyphosate-resistant Italian ryegrass has been confirmed “in a couple of counties in North Carolina.”

According to DREC research associate Robin Bond, Italian ryegrass prefers fertile soil and mild climates. It has a low tolerance for hot, dry climates and harsh winter conditions. Its peak emergence is in the fall but it can emerge during the winter months.

Italian ryegrass germinates in six to 10 days when daytime temperatures are between 50 and 87 degrees and it flowers three weeks after head emergence.

Italian ryegrass is a slow spreader, oftentimes starting out in a corner and slowly taking over. It can also be spread by equipment, animals or flooding. Its color and texture, shiny and dark green, often make it difficult for sensing devices to pick up, making variable-rate applications more difficult.

Robin Bond noted that Italian ryegrass is not as prolific a seed producer as Palmer pigweed, “but it makes 20,000 to 45,000 seeds per plant. Italian ryegrass also exudes an unknown substance that kills surrounding plants, which allows it to be competitive and establish itself as the dominant plant. In rice, this characteristic has been known to reduce yields.”

Italian ryegrass is also a very efficient scavenger of nitrogen.

DREC weed scientist Vijay Nandula said resistance in the weed was first observed in 2005. Italian ryegrass in a field which had been planted to glyphosate-resistant soybeans for six consecutive years survived two applications of glyphosate. The biotypes produced viable seed.

By 2009, scientists confirmed glyphosate-resistant Italian ryegrass in 12 Mississippi counties. Italian ryegrass also has documented resistance to ALS and ACCase herbicides. No resistance has been documented to Command, Dual or Treflan in the United States.

Jeff Ray, a research geneticist with the USDA-ARS, pointed out that Italian ryegrass found in Mississippi shows a high degree of genetic diversity which has the potential to allow adaption to changing environments including what growers are doing to control it.